Possible Clues:

• Indication that men should turn in final job
• Officer about to pass out notes we hear on Armistice Day
• Call for those retiring to carry on with Royal Mail
• Endure letters giving indication it's time to retire
• Correspondent may not want to miss this call to the military
• No more fencing for camp men about to retire?
• Military bugle call
• No more letters after the bugler's farewell
• No more letters tonight, by the sound of it
• Endure job for what you get at the end of the day